Modern communications systems rely on high-data rate servers, base stations and supercomputers. These communications systems can be connected by optical fiber or copper interconnects. Optical fiber interconnects have many advantages compared to copper interconnects, such as: lower loss, lower dispersion, lower cross talk and immunity to electromagnetic interference. In addition, the introduction of microelectronics-based fabrication processes using standard foundry equipment and fabrication steps for the manufacture of silicon photonics will lower the cost of photonic devices. Therefore, fiber optic link deployment is spreading to even the very short reach (VSR) category of systems. While optical fiber interconnects remain a preferred medium to transmit data between nodes, optical fiber links still continue to be more expensive than electrical links due to an expensive packaging step necessitated by optical systems. The challenge of connecting an optical fiber to a semiconductor chip causes the packaging-related expense.
There are many techniques to couple an optical fiber to a semiconductor chip. A first technique is the butt coupling method, which comprises connecting an optical fiber end to an edge of an integrated circuit waveguide. This technique has some limitations since it is only optimal when the cross-section of the waveguide is of similar size to the cross-section of the optical fiber. A size mismatch between the two cross-sections can lead to high signal power losses. The butt coupling method also limits the layout options on the chip since the optical fiber connections are at the edge of the chip. In some instances, a cleaving and a polishing step are added at the end of the butt coupling process, which adds time and expense to the packaging of optical fiber links.
A second technique is the vertical coupling method, which uses grating structures to couple light from a waveguide to an optical fiber. The grating coupler doesn't require any further processing, such as the cleaving and polishing described in butt coupling, and can achieve high coupling efficiency.
However, both types of coupling may require active alignment of the fiber to the waveguide. The Active alignment method relies on the continuous monitoring of the signal strength while attaching the fiber by a technician. This procedure is time intensive, sometimes taking about 60 minutes, which reduces the throughput assembly and increases the cost of the final package.
Some known methods of the prior art are detailed below.
U.S. Pat. No. 6,862,388 discloses the use of a fiber guiding layer (FBL) to avoid space consuming V-grooves, ferrule or fiber brackets. The goal is to increase the vertical coupling port density. The FBL has a funnel shape top to help guide the fiber. While a variety of materials that could be used to make the FBL are described, such as silicon dioxide, sol-gel or polyimide, no clear method of how to make the funnel shape is taught. Furthermore since the structure is vertical, a 45 degree mirror is required if a coupling to a waveguide is considered. Hence the coupling efficiency will be very low if the fiber is placed over a grating coupler.
U.S. Pat. No. 4,744,623 discloses a method of attaching a fiber to a detector on a substrate. The fiber is attached by placing it in an etched cavity and gluing the outside of the fiber cladding to an aluminum layer on top of the wafer with epoxy. The limitation with this technique is that the fiber can only positioned in a vertical manner.
U.S. Pat. No. 6,546,182 discloses a method of making an angled fiber termination. The support assembly consists of a substrate with a through via with a diameter wider than the fiber. A preload on one end of the fiber will result in an angle. The issue with this technique is that the angle heavily depends on many parameters such as the gap between the fiber diameter and the opening in the substrate, the thickness of the substrate and the preload force. The alignment of the termination is also not addressed in this disclosure.
U.S. Pat. No. 7,162,124 discloses a method to connect a fiber to an integrated circuit. The tip of the fiber is cut with an angle. The light reflects at the reflective surface of the angled tip through total internal reflection. This method heavily depends on the cleaving angle of the fiber tip, the alignment with respect to the grating and the roll angle of the fiber.
Therefore, there is a need for an improved connector and method for connecting an optical fiber to a semiconductor device.